Calliphorid hoverfly mimic, Stomorhina lunata

Is this a good year for Stomorhina lunata?  Its larvae develop in the egg pods of locusts, so its occurrence in Britain is likely to be due to adults migrating here from further south.  In most years, I see one or none but this year I have seen it in three locations, all in Surrey:-

1 female, Brookwood Heath, SU956555 on 23/08/2020

1 female, in my Knaphill garden, SU964588, on golden rod flowers, 03/09/2020

20 plus, both sexes, Brick Hill, Chobham Common, SU963645, on 06/09/2020

Brookwood Heath and Brick Hill are both heathland sites and the flies were swept off heather.  Why so many at Brick Hill?  On the same day, sweeping heather on a nearby similar site separated from Brick Hill by the M25 produced no S. lunata.  Sweeping similar habitat on Horsell Common on 7/9/20 also produced no S. lunata.

If S. lunata is a migrant species unable to breed in Britain, how did such an aggregation occur in a small area of Chobham Common?

Andrew Halstead